The present invention relates to light sensors incorporating a charge integrating photodiode as a light transducer.
A light sensor generates an output signal indicating the intensity of light incident upon the light sensor. The light sensor includes a light transducer for converting light into an electrical signal and may also include electronics for signal conditioning, compensation for cross-sensitivities such as temperature, and output signal formatting. Light sensors are used in a wide range of applications including remote sensing, communications, and controls.
One application for light sensors is in automatically dimming vehicle rearview mirrors. Vehicle operators use interior and exterior rearview mirrors to view scenes behind the vehicle without having to face in a rearward direction and to view areas around the vehicle that would otherwise be blocked by vehicle structures. As such, rearview mirrors are an important source of information to the vehicle operator. Bright lights appearing in a scene behind the vehicle, such as from another vehicle approaching from the rear, may create glare in a rearview mirror that can temporarily visually impair or dazzle the operator. This problem is generally worsened during conditions of low ambient light, such as those that occur at night, when the eyes of the vehicle operator have adjusted to the darkness.
Automatically dimming rearview mirrors eliminate the need for the operator to manually switch the mirror. The earliest designs used a single glare sensor facing rearward to detect the level of light striking the mirror. This design proved to be inadequate since the threshold perceived by the operator for dimming the mirror, known as the glare threshold, varied as a function of the ambient light level. An improvement included a second light sensor for detecting the ambient light level. The glare threshold in these systems is based on the amount of ambient light detected. Among the dual sensor designs proposed include those described in U.S. Pat. No. 3,601,614 to Platzer; U.S Pat. No. 3,746,430 to Brean et al.; U.S. Pat. No. 4,580,875 to Bechtel et al.; U.S. Pat. No. 4,793,690 to Gahan et al.; U.S. Pat. No. 4,886,960 to Molyneux et al.; U.S. Pat. No. 4,917,477 to Bechtel et al.; U.S. Pat. No. 5,204,778 to Bechtel; U.S. Pat. No. 5,451,822 to Bechtel et al.; and U.S. Pat. No. 5,715,093 to Schierbeek et al., each of which is incorporated herein by reference.
A key element in the design of an automatic dimming mirror is the type of light transducer used to implement ambient light and glare detection. A primary characteristic of interest in selecting a light transducer type is the dynamic range. The ratio between the intensity of bright sunlight and moonlight is roughly 1,000,000: 1, indicating the wide range that must be sensed by the ambient light sensor. Both the ambient light and the glare light sensors must operate within the ranges of temperature, humidity, shock, and vibration experienced within a vehicle passenger compartment. If a sensor is to be mounted in an outside mirror, even harsher operating conditions can be expected. Sensors and support electronics must also be inexpensive to allow the cost of an automatically dimmed mirror to fall within the range deemed acceptable by an automobile purchaser. Transducers should have good noise immunity or be compatible with noise compensation electronics within the sensor for sensitivity at low light levels. Transducers should further have a spectral response similar to the frequency response of the human eye. As a final desirable characteristic, the sensor must be easily integratable into the types of digital control systems commonly found in automotive applications.
Photodiode light sensors incorporate a silicon-based photodiode and conditioning electronics on a single substrate. The photodiode generates charge at a rate proportional to the amount of incident light. This light-induced charge is collected over an integration period. The resulting potential indicates the level of light to which the sensor is exposed over the integration period. Light sensors with integral charge collection have many advantages. By varying the integration time, the sensor dynamic range is greatly extended. Also, the ability to incorporate additional electronics on the same substrate as the photodiode increases noise immunity and permits the sensor output to be formatted for use by a digital circuit. Component integration additionally reduces the system cost. Silicon light sensors are relatively temperature invariant and can be packaged to provide the necessary protection from humidity, shock, and vibration. One disadvantage of silicon-based light transducers is a frequency response different from that of the human eye. A variety of charge integrating photodiode devices have been described including those in U.S. Pat. No. 4,916,307 to Nishibe et al.; U.S. Pat. No. 5,214,274 to Yang; U.S. Pat. No. 5,243,215 to Enomoto et al.; U.S. Pat. No. 5,338,691 to Enomoto et al.; and U.S. Pat. No. 5,789,737 to Street, each of which is incorporated herein by reference.
One difficulty with all types of light sensors is the occurrence of operating anomalies at high temperatures. Some devices become extremely non-linear at high temperatures. Some devices may suffer a permanent change in operating characteristics. Devices may even provide completely false readings such as indicating bright light in low light conditions due to excessive thermal noise. Traditionally, the only way to deal with this problem has been to incorporate a temperature sensor and associated electronics into systems that use light sensors.
What is needed is a light sensor with a wide dynamic range that may be incorporated into cost sensitive digital systems such as automatically dimming rearview mirrors. The light sensor should compensate for temperature cross-sensitivity and, preferably, provide an indication of light sensor temperature. A charge integrating light sensor having an externally determined integration period is also desirable.